Hardenable composition with a particular combination of characteristics

ABSTRACT

A curable epoxy resin composition comprising 
     (a) a cycloaliphatic epoxy resin that is liquid at RT and, suspended therein, a core/shell polymer, 
     (b) a polycarboxylic anhydride and 
     (c) fillers, 
     wherein the composition is flame-retardant because two different fillers (c1) and (c2) are present, the nature of filler (c1) being such that, starting at RT, it is able to release water as the temperature rises, the total proportion of fillers (c1) and (c2) is from 58 to 73% by weight, based on the total amount of components (a), (b), (c1) and (c2), and the ratio by weight of the fillers (c1):(c2) is from 1:3 to 1:1, is suitable as a casting resin, especially in the impregnation of electrical coils and in the production of electrical components, such as air-cooled transformers, bushings, insulators, switches, sensors, converters and cable end seals.

The present invention relates to curable compositions, to their use, forexample as casting resins in the production of air-cooled transformersand other electrical components, and to the crosslinked productsobtainable by curing the compositions, which products are distinguishedby the fact that they exhibit simultaneously the features of good flameretardance, high mechanical strength and low dielectric losses at highoperating temperatures.

Air-cooled transformers (voltage range up to about 40 kV) are providedwith a winding, the sheathing of which consists of an electricallyinsulating synthetic resin. In addition to providing insulation, thesynthetic resin sheathing should also contribute to the mechanicalstrength of the windings and also have flame-retardant properties.

The critical factors for a sheathing resin for high performancetransformers are the oxygen index for combustibility, the temperature atwhich the dielectric loss factor tan 6 is 25% at 50 Hz and the crackindex value achieved, which is a measure of resistance to temperaturevariation.

Flame-retardant casting resins for potting air-cooled transformers arewell known and are generally based on bisphenol A epoxy resins,reinforcing fillers and flame-retardants. For example, U.S. Pat. No.3,202,947 describes flame-retardant compositions for air-cooledtransformers, containing liquid bisphenol A diglycidyl ethers,hexahydrophthalic acid, hydrated alumina and tris(chloroalkyl)phosphates.

Cycloaliphatic resin systems are also known. U.S. Pat. No. 4,009,141describes electrically insulating curable compositions consisting ofselected cycloaliphatic epoxy resins and dicarboxylic anhydrides, whichare reinforced with large amounts of zirconium silicate fillers andcontain finely divided hydrated alumina as additional second filler.They are suitable for the encapsulating insulation of electricalcomponents, such as, for example, of metal transformer components, ortransformer bushings.

Curable, flame-retardant compositions for air-cooled transformers arealso described in FR 2 630 578 B1. Those compositions contain at least20% by weight pretreated aluminium hydroxide, based on the totalcomposition consisting of resin, hardener and reinforcing additives.“Pretreated” in this context means that, by means of heat treatment,water is removed from the aluminium hydroxide in an amount of about from0.5 to 10% by weight, based on the original weight before removal of thewater.

Since in such systems the dielectric loss factor tan δ increasesconsiderably at higher temperature, those systems are not suitable fortransformers having high operating temperatures.

There is therefore a need for casting resin formulations that exhibitsimultaneously the features of flame retardance, low dielectric lossesand good mechanical properties, especially good cracking behaviour.

That problem has now been solved by the use of cycloaliphatic systemscomprising core/shell polymers, as described in EP 0 578 613 A2. It hasbeen found that the addition of certain fillers in certain ratios andcertain amounts yields potting compounds that are distinguished both bylow brittleness and by a low tan δ value and also by good flameretardance.

The present invention accordingly relates to curable compositionscomprising

(a) a cycloaliphatic epoxy resin that is liquid at RT and, suspendedtherein, a core/shell polymer,

(b) a polycarboxylic anhydride and

(c) fillers,

wherein the composition is flame-retardant because two different fillers(c1) and (c2) are present, the nature of filler (C1) being such that,starting at RT, it is able to release water as the temperature rises,the total proportion of fillers (C1) and (c2) is from 58 to 73% byweight, based on the total amount of components (a), (b), (C1) and (c2),and the ratio by weight of the fillers (C1):(c2) is from 1:3 to 1:1.

The compositions according to the invention are resin systems ofmoderate to relatively high viscosity that can be fully cured by heat.In the cured state they are thermosetting materials of relatively highrigidity having a glass transition temperature of about from 80 to 140°C. The term “cycloaliphatic epoxy resin” in the context of thisinvention denotes any epoxy resin having cycloaliphatic structuralunits, that is to say it includes both cycloaliphatic glycidyl compoundsand β-methylglycidyl compounds-as well as epoxy resins based oncycloalkylene oxides. “Liquid at room temperature (RT)” is to beunderstood as meaning pourable compounds that are liquid at 25° C., i.e.are of low to medium viscosity (viscosity less than about 20 000 mPa·s).

Suitable cycloaliphatic glycidyl compounds and β-methylglycidylcompounds are the glycidyl esters and β-methylglycidyl esters ofcycloaliphatic polycarboxylic acids, such as tetrahydrophthalic acid,4-methyltetrahydrophthalic acid, hexahydrophthalic acid,3-methylhexahydrophthalic acid and 4-methylhexahydrophthalic acid.

Further suitable cycloaliphatic epoxy resins are the diglycidyl ethersand β-methylglycidyl ethers of cycloaliphatic alcohols, such as1,2-dihydroxycyclohexane, 1,3-dihydroxycyclohexane and1,4-dihydroxycyclohexane, 1,4-cyclohexanedimethanol,1,1-bis(hydroxymethyl)cyclohex-3-ene, bis(4-hydroxycyclohexyl)methane,2,2-bis(4-hydroxycyclohexyl)propane and bis(4-hydroxycyclohexyl)sulfone.

Examples of epoxy resins having cycloalkylene oxide structures arebis(2,3-epoxycyclopentyl) ether, 2,3-epoxycyclopentylglycidyl ether,1,2-bis(2,3-epoxycyclopentyl)ethane, vinyl cyclohexene dioxide,3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate,3,4-epoxy-6-methylcyclohexylmethyl3′,4′-epoxy-6′-methylcyclohexanecarboxylate,bis(3,4-epoxycyclohexylmethyl) adipate andbis(3,4-epoxy-6-methylcyclohexylmethyl) adipate.

Preferred cycloaliphatic epoxy resins arebis(4-hydroxycyclohexyl)methanediglycidyl ether,2,2-bis(4-hydroxycyclohexyl)propanediglycidyl ether, tetrahydrophthalicacid diglycidyl ester, 4-methyltetrahydrophthalic acid diglycidyl ester,4-methylhexahydrophthalic acid diglycidyl ester,3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate andespecially hexahydrophthalic acid diglycidyl ester.

The cycloaliphatic epoxy resins can also be used in combination withaliphatic epoxy resins. As “aliphatic epoxy resins” it is possible touse epoxidation products of unsaturated fatty acid esters. It ispreferable to use epoxy-containing compounds derived from mono- andpoly-fatty acids having from 12 to 22 carbon atoms and an iodine numberof from 30 to 400, for example lauroleic acid, myristoleic acid,palmitoleic acid, oleic acid, gadoleic acid, erucic acid, ricinoleicacid, linoleic acid, linolenic acid, elaidic acid, licanic acid,arachidonic acid and clupanodonic acid.

For example, there are suitable the epoxidation products of soybean oil,linseed oil, perilla oil, tung oil, oiticica oil, safflower oil,poppyseed oil, hemp oil, cottonseed oil, sunflower oil, rapeseed oil,polyunsaturated triglycerides, triglycerides from euphorbia plants,groundnut oil, olive oil, olive kernel oil, almond oil, kapok oil,hazelnut oil, apricot kernel oil, beechnut oil, lupin oil, maize oil,sesame oil, grapeseed oil, lallemantia oil, castor oil, herring oil,sardine oil, menhaden oil, whale oil, tall oil and derivatives thereof.

Also suitable are higher unsaturated derivatives that can be obtained bysubsequent dehydrogenation reactions of those oils.

The olefinic double bonds of the unsaturated fatty acid radicals of theabove-mentioned compounds can be epoxidised in accordance with knownmethods, for example by reaction with hydrogen peroxide, optionally inthe presence of a catalyst, an alkyl hydroperoxide or a peracid, forexample performic acid or peracetic acid. Within the scope of theinvention, both the fully epoxidised oils and the partially epoxidisedderivatives that still contain free double bonds can be used forcomponent (a).

Preference is given to the use of epoxidised soybean oil and epoxidisedlinseed oil.

When cycloaliphatic epoxy resins are used in combination with aliphaticepoxy resins, the advantageous ratio by weight of cycloaliphatic toaliphatic component is from 1:0 to 0.6:0.4.

The cycloaliphatic epoxy resins used according to the invention compriseso-called core/shell polymers in suspended form as tougheners, thetougheners being liquid or solid in the starting state. They should notcontain reactive groups that could react with the epoxy groups of theepoxy resin in question. It is preferable to use solid tougheners. Theyhave the advantage that the particle size and also the proportion oftoughening phase in the suspension are preset, whereas in the case ofliquid tougheners the required second phase is formed during the curingwith the epoxy resin.

Core/shell polymers generally have a soft core of an elastomericmaterial that is insoluble in the epoxy resin. Grafted onto that core isa shell of polymeric material that does not contain any groups capableof reacting with the epoxy resin.

Examples of elastomers that can be used as core material arepolybutadiene, polyacrylic acid esters and polymethacrylic acid estersand co- or ter-polymers thereof with polystyrene, polyacrylonitrile orpolysulfide.

Examples of polymeric shell materials are polystyrene,polyacrylonitrile, polyacrylate and methacrylate homo-, di- orter-polymers and styrene/acrylonitrile/glycidyl methacrylateterpolymers.

Preference is given to suspensions comprising a solid core/shellpolymer.

The size of such core/shell particles is advantageously from 0.05 to 30μm, preferably from 0.05 to 15 μm. Core/shell particles less than 1 μmin size are especially used.

The core/shell polymers can be produced, for example, in the mannerdescribed in U.S. Pat. No. 4,419,496 or EP-A 0 045 357.

Especially preferred is the use of core/shell polymers that contain acore of polybutadiene or polybutadiene/polystyrene. Such a core materialis preferably only partially crosslinked. Further core materials arepolyacrylates and polymethacrylates, especially polyacrylic acid estersand polymethacrylic acid esters and di- or ter-polymers thereof.

The shell consists especially of polymers based on methyl methacrylate,cyclohexyl methacrylate, butyl acrylate, styrene and methacrylonitrile,but especially based on polymethyl methacrylate.

The amount of toughener in the suspensions according to the inventionthat comprise a cycloaliphatic or aliphatic epoxy resin is preferablyfrom 1 to 30% by weight, especially from 5 to 10% by weight, based onthe epoxy resin.

For curing the compositions according to the invention, polycarboxylicanhydrides are used.

They may be linear aliphatic polymeric anhydrides, for examplepolysebacic polyanhydride or polyazelaic polyanhydride, or cycliccarboxylic anhydrides.

Cyclic carboxylic anhydrides are especially preferred.

Examples of cyclic carboxylic anhydrides are:

succinic anhydride, citraconic anhydride, itaconic anhydride,alkenyl-substituted succinic anhydrides, dodecenylsuccinic anhydride,maleic anhydride and tricarballylic anhydride, a maleic anhydride adductwith cyclopentadiene or methylcyclopentadiene, a linoleic acid adductwith maleic anhydride, alkylated endoalkylenetetrahydrophthalicanhydrides, methyltetrahydrophthalic anhydride and tetrahydrophthalicanhydride, the isomeric mixtures of the two latter compounds beingespecially suitable. Especially preferred are hexahydrophthalicanhydride and methylhexahydrophthalic anhydride.

The compositions according to the invention can optionally additionallycomprise a curing accelerator. Suitable accelerators are known to theperson skilled in the art. Examples that may be mentioned are:

complexes of amines, especially tertiary amines, with boron trichlorideor boron trifluoride;

tertiary amines, such as benzyldimethylamine;

urea derivatives, such as N-4-chlorophenyl-N′,N′-dimethylurea (monuron);

unsubstituted or substituted imidazoles, such as imidazole or2-phenylimidazole.

Preferred accelerators are tertiary amines, especiallybenzyldimethylamine, and imidazoles (e.g. 1-methylimidazole) for theabove-mentioned compositions that comprise epoxidised oils.

The curing agents and, where applicable, accelerators are used in thecustomary effective amounts, that is to say amounts sufficient forcuring the compositions according to the invention. The ratio of thecomponents resin system/hardener/accelerator depends upon the nature ofthe compounds used, the rate of curing required and the propertiesdesired in the end product and can readily be determined by the personskilled in the art. Generally, from 0.4 to 1.6 equivalents, preferablyfrom 0.8 to 1.2 equivalents, of anhydride groups per epoxy equivalentare used.

The curing accelerators are usually used in amounts of from 0.1 to 20parts by weight per 100 parts by weight of epoxy resin.

As component c1 the compositions according to the invention comprisefillers having flame-retardant properties. Such fillers exhibitflame-retardant properties because their nature is such that, startingat room temperature, they are able to release water as the temperaturerises. There are therefore suitable, for example, aluminium hydroxide,water-containing magnesia or zinc borate or other substances thatdecompose with the release of water at elevated temperatures.

It is preferable to use aluminium hydroxide, which may be eitheruntreated or thermally pretreated and/or silanised Al(OH)₃. “Thermallypretreated” in this context means that, by means of heat treatment,water is removed from the aluminium hydroxide advantageously in anamount of from about 0.5 to about 10% by weight, based on the originalweight before removal of the water. Methods in this connection aredescribed in FR 2 630 578 B1.

In order to obtain the desired mechanical strength, the starting resinis actively reinforced by the addition of a further filler c2 which isdifferent from c1. Examples of suitable reinforcing materials c2 areglass fibres or carbon fibres. The following materials, for example,also come into consideration as component c2: metal powder, wood flour,glass powder, glass beads, semi-metal and metal oxides, such as SiO₂(quartz sand, quartz powder, silanised quartz powder, fused silicapowder, silanised fused silica powder), aluminium oxide, titanium oxideand zirconium oxide, semi-metal and metal nitrides, for example siliconnitride, boron nitrides and aluminium nitride, semi-metal and metalcarbides (SiC and boron carbides), metal carbonates (dolomite, chalk,CaCO₃), metal sulfates (barytes, gypsum), ground minerals, and naturalor synthetic minerals chiefly of the silicate series, e.g. zeolites(especially molecular sieves), talcum, mica, kaolin, wollastonite,silanised wollastonite and others.

Preferred fillers c2 are quartz powder, silanised quartz powder,wollastonite and silanised wollastonite, both on their own and incombination.

Wollastonite is a naturally occurring acicular calcium silicate of theformula Ca₃[Si₃O₉] having particle sizes in the micron range.Artificially produced wollastonite is also acicular. Wollastonite iscommercially available, for example under the name Nyad® from the Nycocompany.

The total proportion of components (c1) and (c2) in % by weight is from58 to 73% by weight, preferably from 63 to 68% by weight, based on thetotal amount of components (a), (b), (C1) and (c2), and the ratio byweight of the fillers (C1):(c2) is from 1:3 to 1:1, preferably from1:2.3 to 1:2.

If desired, in addition to fillers c1 and c2 it is also possible to usea wetting and dispersing agent, which reduces the mainly electrostaticinteractive forces between resin and filler and the increased viscositycaused thereby.

The wetting and dispersing agent is advantageously used in an amount ofabout from 0.1 to 2.0% by weight, based on the total amount ofcomponents (a) and (b).

In addition to the fillers c1 and c2 mentioned above and, whereapplicable, a wetting and dispersing agent, the curable mixtures maycomprise further customary additives, e.g. antioxidants, lightstabilisers, fillers containing water of crystallisation, plasticisers,dyes, pigments, fungicides, thixotropic agents, antifoams, antistatics,lubricants, anti-settling agents, wetting agents and mould-release aids.

The compositions according to the invention can be produced inaccordance with known methods using known mixing apparatus, for examplestirrers, kneaders and rollers. The curing of the mixtures according tothe invention can be carried out in known manner in one or more steps.It is generally effected by heating to temperatures of from 60° C. to200° C., especially from 80° C. to 180° C. When curing is carried out intwo or more steps, it means that curing is effected in stages, each at ahigher temperature.

The invention accordingly relates also to crosslinked productsobtainable by curing a composition according to the invention.

EXAMPLES

The preparation, composition and test results of four Reference Examplesand three Invention Examples are given below. For the correspondingquantitative data see also Table 1. In each case a resin premix and ahardener premix are prepared. The two mixtures are then combined to forma total mixture, which is used in the casting, full curing andmeasurement of test specimens. The general steps in detail:

Resin Premix

For each resin premix, all the components are introduced into a mixingvessel and intimately mixed together for one hour at a pressure of 1mbar and a temperature of 50° C.

Hardener Premix

Likewise, for each hardener premix, all the components are introducedinto a mixing vessel and intimately mixed together for one hour at apressure of 1 mbar and a temperature of 50° C.

Total Premix

For the preparation of each total mixture, the respective resin andhardener premixes are intimately mixed together for 20 min at a pressureof 1 mbar and a temperature of 50° C.

Curing/test Specimens

For the production of the test specimens, each total mixture isintroduced into a metal mould that has been preheated to 100° C. andthen fully cured, first for 2 hours at 100° C. and then for 10 hours at140° C. Using the test specimens so produced, the measurement valuesgiven in Table 1 are obtained.

The resins and hardeners listed below, which are used for the Examples,all originate from Ciba Spezialitätenchemie:

Resins (Data: E=Epoxy Content in Equivalents/kg, V=Viscosity in mPa·s)

1=liquid cycloaliphatic epoxy resin based on hexahydrophthalic aciddiglycidyl ester; E: 5.8-6.1; V: 700-1000

2=liquid mixture of 90% by weight bisphenol A epoxy resin andpolyoxypropylene glycol diglycidyl ether; E: 4.9-5.4; V: 4200-5700

3=bisphenol A epoxy resin; E: 5.1-5.3; V: 8500-15 000

4=liquid cycloaliphatic epoxy resin as resin 1 but with 10% core/shellcontent based on methyl methacrylate and polybutadiene latex (1:1); E:5.2-5.5; V: 3500-5000

Hardeners

1=hexahydrophthalic anhydride

2=amine-accelerated hardener formulation based on(methylhexahydrophthalic anhydride/hexahydrophthalicanhydride/methyltetrahydrophthalic anhydride=70:15:15 pbw), containing38% by weight of a semiester of succinic acid and polyethylene glycol(400 g/mol)

3=accelerated formulation of methyltetrahydrophthalic anhydride and 50%by weight semiester of tetrahydrophthalic acid and polypropylene glycol(400 g/mol), with an anhydride equivalent of from 2.6 to 3 eq./kg;

4=mixture of hexahydrophthalic anhydride and methylhexahydrophthalicanhydride (70/30 ppw)

Other components used: antifoam: “BYK ® A 501” (Byk Chemie) dispersant:“BYK ® W 9010” (Byk Chemie) silicic acid: “R 202” (Degussa) accelerator1: N,N-dimethylbenzylamine accelerator 2: 10% methylimidazole, 3.3%NaOCH₃, 7.7% CH₃OH, balance: PPG 400 quartz powder: “W 12” (QuarzwerkeFrechen) wollastonite: untreated, natural wollastonite, K = averageparticle size (D₅₀ value) in μ: 13-20; specific surface area (BET):1-1.4 m²/g anti-settling agent: “EXL 2300” (Rohm & Haas)

aluminium hydroxides (ATH=aluminium trihydroxide):

Data: K=average particle size (D₅₀ value) in μ, G=loss on ignition in %

ATH1: heat-treated, silanised; K: 14-18, G: 31.0 +/− 1 ATH2: untreated,“Apyral 2E” (Nabaltec); K: 15-27; G: 34.5 +/− 1 ATH4: untreated, “Apyral4” (Nabaltec); K: 9-13; G: 34.5 +/− 1

A) REFERENCE EXAMPLES (NOT ACCORDING TO THE INVENTION) A1) Example of aNon-flame-retarded Cycloaliphatic System

a) Resin premix (1 filler)

100 g resin 1; 150 g quartz powder.

b) Hardener premix (1 filler)

90 g hardener 1; 150 g quartz powder, 3 g accelerator 2.

c) Total mixture (total filler content 61%)

250 g resin premix; 243 g hardener premix.

A2) Example of a Typical Flame-retarded System

a) Resin premix (2 fillers in a ratio of 1:4)

100 g resin 2; 39 g quartz powder; 156 g ATH2.

b) Hardener premix (2 fillers in a ratio of 1:4)

100 g hardener 2; 39 g quartz powder; 156 g ATH2.

c) Total mixture (total filler content 66%)

295 g resin premix; 295 g hardener premix.

A3) Experiment for a Flame-retarded Cycloaliphatic System Having GoodDielectric Properties by the Addition of Aluminium Hydroxide to SystemA1

a) Resin premix (2 fillers in a ratio of 1:4)

100 g resin 1; 30 g quartz powder; 120 g ATH2.

b) Hardener premix (2 fillers in a ratio of 1:4)

90 g hardener 1; 30 g quartz powder; 120 g ATH2, 3 g accelerator 2.

c) Total mixture (total filler content 61%)

250 g resin premix; 243 g hardener premix.

A4) Formulation According to FR 2 630 578 B1

a) Resin premix (2 fillers in a ratio of 1:3)

200 g resin 3; 75 g quartz powder; 225 g ATH1.

b) Hardener premix (2 fillers in a ratio of 1:3)

200 g hardener 3; 75 g quartz powder; 225 g ATH1.

c) Total mixture (total filler content 60%)

500 g resin premix; 500 g hardener premix.

B) INVENTION EXAMPLES B1)

a) Resin premix (2 fillers in a ratio of 2.33:1)

175.8 g resin 4; 261.5 g quartz powder; 112.1 g ATH1.

b) Hardener premix (2 fillers in a ratio of 2.33:1)

140.9 g hardener 4; 2.55 g dispersant; 0.72 g accelerator 1; 214.5 gquartz powder;

91.9 g ATH1.

c) Total mixture (total filler content 68%)

549.4 g resin premix; 450.6 g hardener premix.

B2)

a) Resin premix (2 fillers in a ratio of 1.86:1)

400.9 g resin 4; 1 g antifoam; 1 g silicic acid; 388.1 g wollastonite;209 g ATH1.

b) Hardener premix (2 fillers in a ratio of 1.86:1)

320.6 g hardener 4; 5 g dispersant; 1.4 g accelerator 1; 8 g EXL 2300;

2 g silicic acid; 432.9 g wollastonite; 232 g ATH1.

c) Total mixture (total filler content 63%)

1000 g resin premix, 1001.9 g hardener premix.

B3) As B1 but Using ATH4 Instead of ATH1

a) Resin premix (2 fillers in a ratio of 2.33:1)

175.8 g resin 4; 261.5 g quartz powder; 112.1 g ATH4.

b) Hardener premix (2 fillers in a ratio of 2.33:1)

140.9 g hardener 4; 2.55 g dispersant; 0.72 g accelerator 1; 214.5 gquartz powder;

91.9 g ATH4.

c) Total mixture (total filler content 68%)

549.4 g resin premix; 450.6 g hardener premix.

TABLE 1 Reference/Invention Examples RE1 RE2 RE3 RE4 IE1 IE2 IE3 resin 12 1 3 4 4 4 cycloaliphatic yes no yes no yes yes yes [g] 100 100 100 200175.8 400.9 175.8 core/shell in resin no no no no yes yes yes hardener 12 1 3 4 4 4 [g] 90 100 90 200 140.9 320.6 140.9 fillers [g] quartz (Q)or Q Q Q Q Q W Q wollastonite (W) 300 78 60 150 476 821 476 ATH1 — — —450 204 441 — ATH2 — 312 240 — — — — ATH4 — — — — — — 204 content of ATHin 0 80 80 75 30 35 30 filler [%] total filler content 61.0 66.0 61.060.0 68.0 63.0 68.0 [%] Measurement values Tg (DSC) [° C.]¹⁾ 105 60 11263 102 102 107 elongation 1.8 0.65 0.74 1.1 0.77 0.8 0.81 (tensile test)[%]²⁾ G_(1C) [J/m²]³⁾ 525 405 402 542 432 648 490 CTE [10⁻⁶/K]⁴⁾ 40 4038.4 41 36.9 33.9 33.8 temperature for 155 105 135 ˜100 167 163 175 tanδ = 25% [° C.]⁵⁾ (10%) LOI (oxygen index) 25 40 37.9 29.9 30.4 30.1 31.3[%]⁶⁾ crack index⁷⁾ −43 −7 +5 −37 −26 −50 −34 Note: ¹⁾DSC (DifferentialScanning Calorimetry) carried out using TA 4000 apparatus (Mettler)²⁾according to ISO R527 ³⁾breaking energy G_(1C): double torsion test⁴⁾CTE = Coefficient of Thermal Expansion measured according to DIN 53752⁵⁾electrical values (tan δ) according to DIN 53483, measurementfrequency 50 Hz ⁶⁾LOI according to ASTM D2863 ⁷⁾see explanation in text

Reference Example 1 is a non-flame-retarded cycloaliphatic system.Reference Example 2 is a typical flame-retarded system. ReferenceExample 3 is the experiment to obtain a flame-retarded cycloaliphaticsystem having good dielectric properties by the addition of aluminiumhydroxide to the system of Reference Example 1. Finally, ReferenceExample 4 is a formulation according to FR 2 630 578 B1.

The crack index given in Table 1 represents a number of mechanicalvalues, all of which are able to influence the cracking behaviour in theevent of stress caused by changes in temperature, combined to form asingle variable, the so-called crack index. The crack index enables anobjective comparison of the mechanical qualities of different systems tobe made more easily.

The following qualitative statements, relating to the effects of changesin individual parameters on the temperature variation behaviour, can bederived empirically:

1. The higher the T_(G) value, the poorer is the temperature variationbehaviour.

2. The lower the G_(1C) value, the poorer is the temperature variationbehaviour.

3. The lower the elongation value, the poorer is the temperaturevariation behaviour.

4. The higher the expansion coefficient (CTE), the poorer is thetemperature variation behaviour.

However, when several parameters are changed simultaneously it is nolonger possible to make qualitative statements about the resultingtemperature variation behaviour. When, for example, both the T_(G) valueand the G_(1C) value rise but the CTE value decreases, it is no longerpossible to make predictions as to the temperature variation behaviourthat is to be expected.

At Ciba Spezialitättenchemie, statistical evaluations of measurementvalues using a large number of extremely varied systems have led to aformula for calculating a new variable, the so-called crack index, whichformula is extremely helpful from the standpoint of applicationtechnology. Using this variable it is now possible, even in the event ofmultiple changes, to make statements relating to the resultanttemperature variation behaviour to be expected.

The formula is as follows:

RI=−498.08·Z ^(0.18480890) ·G ^(0.194114601)·(A−18)^(−0.391334273) ·T^(−0.158387791)+224.25

where: RI=crack index; Z=tensile elongation in %; G=G_(1C) in J/m²;

A=expansion coefficient in ppm/K and T=T_(G) in ° C.

Using that formula, a decrease in the crack index value indicates animprovement in the resistance to temperature variation that is to beexpected. The crack index correlates very well with the average crackingtemperature in ° C., which can be determined in a practical crackingtest. That cracking temperature and thus also the crack index provide anindication of the temperature beyond which cracks are likely to form(caused by stress in the event of temperature fluctuations anddifferences in the expansion coefficients of potting compound and metalinsert). Air-cooled transformers subjected to a temperature change testaccording to Cenelec standard HD 464 (which begins at −25° C. for class“C2”) have a good chance of withstanding the test when the pottingsystem used has a crack index below −25 (the value critical for thisinvention). The lower the crack index, the less susceptible is thesystem in practice in respect of stress.

Table 1 also shows the measured LOI values (according to ASTM D2863).The higher the LOI value, the better is the flame retardance. For-ausable system that can be said to be sufficiently flame-retardant, themeasured LOI value should be >30.

Finally, Table 1 gives the measured temperatures at which the dielectricloss factor tan δ has a certain value (here 25%). The higher thetemperature at which the tan δ has that value, the better is thedielectric behaviour of the material at elevated temperature. The aim ofthe present invention was, in view of the intended applications, toobtain values of appreciably >150° C.

The compositions according to the invention are distinguished by thefact that they exhibit simultaneously the features of good flameretardance, high mechanical strength and low dielectric losses at highoperating temperatures.

They are therefore particularly suitable as casting resins especiallyfor applications in the electronics industry, for example in theimpregnation of electrical coils and in the production of air-cooledtransformers, bushings, insulators, switches, sensors, converters andcable end seals.

The invention relates also to the use of the compositions according tothe invention as electrical insulation material.

What is claimed is:
 1. A curable composition comprising: (a) a cycloaliphatic epoxy resin that is liquid at room temperature and, suspended therein, a core/shell polymer; (b) a polycarboxylic anhydride; and (c) two different fillers, (c1) and (c2), wherein (c1) is a filler that is able to release water as the temperature rises above room temperature; (c2) is a reinforcing material; the total proportion of fillers (c1) and (c2) is from 58 to 73% by weight based on the total amount of components (a), (b), (c1) and (c2) in the composition; and the ratio by weight of the fillers (c1):(c2) is in the range from 1:3 to 1:1.
 2. A curable composition according to claim 1, wherein the total proportion of fillers (c1) and (c2) is from 62 to 68% by weight.
 3. A curable composition according to claim 2, wherein the ratio by weight of the fillers (c1):(c2) is in the range from 1:2.3 to 1:2.
 4. A curable composition according to claim 2, wherein the ratio by weight of the fillers (c1):(c2) is 1:1.86.
 5. A curable composition according to claim 2, wherein the ratio by weight of the fillers (c1):(c2) is 1:2.33.
 6. A curable composition according to claim 1, wherein the cycloaliphatic epoxy resin is selected from the group consisting of bis(4-hydroxycyclohexyl)methanediglycidyl ether, 2,2-bis(4-hydroxycyclehexyl)propanediglycidyl ether, tetrahydrophthalic acid diglycidyl ester, 4-methyltetrahydrophthalic acid diglycidyl ester, 4-methylhexahydrophthalic acid diglycidyl ester, and 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate.
 7. A curable composition according to claim 1, wherein the cycloaliphatic epoxy resin is hexahydrophthalic acid diglycidyl ester.
 8. A curable composition according to claim 1, wherein the core/shell polymer is a solid core/shell polymer.
 9. A curable composition according to claim 1, wherein the amount of core/shell polymer suspended in the epoxy resin is from 1 to 30% by weight based on the epoxy resin.
 10. A curable composition according to claim 9, wherein the amount of core/shell polymer suspended in the epoxy resin is from 5 to 10% by weight based on the epoxy resin.
 11. A curable composition according to claim 1, wherein the core/shell polymer is methylmethacrylate and polybutadiene latex in a 1:1 ratio.
 12. A curable composition according to claim 1, wherein filler (c2) is selected from the group consisting of quartz powder, silanised quartz powder, wollastonite, silanised wollastonite, and combinations thereof.
 13. A curable composition according to claim 1, wherein filler (c1) is selected from the group consisting of untreated or thermally pretreated aluminum hydroxide, and silanised versions of the foregoing.
 14. A crosslinked product obtained by curing a composition according to claim
 1. 15. An improved method for the impregnation of electrical coils with a curable casting resin, wherein the improvement comprises the use of a curable composition according to claim 1 as the casting resin.
 16. An improved method for the production of an electrical component utilizing a curable casting resin, wherein the improvement comprises the use of a curable composition according to claim 1 as the casting resin.
 17. The improved method of claim 16, wherein the electrical component is selected from the group consisting of air-cooled transformers, bushings, insulators, switches, sensors, converters, and cable end seals. 